Passively folding propeller blades for drag reduction

ABSTRACT

A propulsion unit includes a motor rotor that spins about a central rotational axis, propeller blades each having a proximal base and a distal tip, and pivot mounts each coupling the proximal base of a corresponding one of the propeller blades to the motor rotor. The propeller blades each freely pivot at the proximal base about a corresponding offset pivoting axis that is substantially parallel to but offset from the central rotational axis of the motor rotor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/663,500, filed Apr. 27, 2018, which is hereby incorporated byreference in its entirety. The present application is also related toU.S. application Ser. No. 16/007,814 filed on Jun. 13, 2018 andentitled, “Clip-On Propeller Mount.

TECHNICAL FIELD

This disclosure relates generally to propeller blade mounts, and inparticular but not exclusively, relates to propeller blade mounts forunmanned aerial vehicles.

BACKGROUND INFORMATION

An unmanned vehicle, which may also be referred to as an autonomousvehicle, is a vehicle capable of travel without a physically-presenthuman operator. An unmanned vehicle may operate in a remote-controlmode, in an autonomous mode, or in a partially autonomous mode.

When an unmanned vehicle operates in a remote-control mode, a pilot ordriver that is at a remote location can control the unmanned vehicle viacommands that are sent to the unmanned vehicle via a wireless link. Whenthe unmanned vehicle operates in autonomous mode, the unmanned vehicletypically moves based on pre-programmed navigation waypoints, dynamicautomation systems, or a combination of these. Further, some unmannedvehicles can operate in both a remote-control mode and an autonomousmode, and in some instances may do so simultaneously. For instance, aremote pilot or driver may wish to leave navigation to an autonomoussystem while manually performing another task, such as operating amechanical system for picking up objects, as an example.

Various types of unmanned vehicles exist for various differentenvironments. For instance, unmanned vehicles exist for operation in theair, on the ground, underwater, and in space. Unmanned aerial vehicles(UAVs) or drones are becoming more popular in general. As their designsare refined and their capabilities expanded, their suitability forcommercial use is expected to expand. Designs that improve theefficiency and endurance of UAVs will expand their mission capabilities.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified. Not all instances of an element arenecessarily labeled so as not to clutter the drawings where appropriate.The drawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles being described.

FIG. 1 is a perspective view illustration of an aerial vehicle withpropeller blades of vertical propulsion units in a deployed position, inaccordance with an embodiment of the disclosure.

FIG. 2 is a plan view illustration of an aerial vehicle with propellerblades of vertical propulsion units in a stowed position, in accordancewith an embodiment of the disclosure.

FIG. 3 is a perspective view illustration of a subassembly for couplingpropeller blades to a motor rotor including a clip-in rotor cap thatmates to a clip-in base mount, in accordance with an embodiment of thedisclosure.

FIG. 4 is a perspective view illustration of the subassembly includingan underside view of the clip-in rotor cap, in accordance with anembodiment of the disclosure.

FIG. 5 is an exploded view illustration of the subassembly including theclip-in rotor cap, in accordance with an embodiment of the disclosure.

FIG. 6A is a perspective view illustration showing how the clip-in rotorcap attaches to the clip-in base mount, in accordance with an embodimentof the disclosure.

FIG. 6B is a perspective view illustration of the clip-in base mount, inaccordance with an embodiment of the disclosure.

FIG. 7 is a perspective view illustration of a subassembly for couplingpropeller blades to a motor rotor, in accordance with an embodiment ofthe disclosure.

FIGS. 8A and 8B are perspective view illustrations of a holder cap, inaccordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Embodiments of an apparatus, system, and method of operation forpassively folding propeller blades to reduce drag resistance aredescribed herein. In the following description numerous specific detailsare set forth to provide a thorough understanding of the embodiments.One skilled in the relevant art will recognize, however, that thetechniques described herein can be practiced without one or more of thespecific details, or with other methods, components, materials, etc. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Embodiments disclosed herein describe mechanical structures andtechniques for pivot mounting propeller blades to a motor rotor thatenable passive folding of the propeller blades from a deployed positionwhen in use to a stowed position when idle. The stowed position providesreduced drag during forward flight. The folding of the propeller bladesto the stowed position is passively achieved by wind resistance whilepivoting back to the deployed position is achieved via centrifugal forcedue to offset positioning of pivot mounts from the central rotationalaxis of the motor rotor.

While the propeller blade mounts are applicable for use in a variety ofvehicle applications, the described techniques are particularly usefulin applications having separate propulsion units for horizontal andvertical propulsion where the vertical takeoff and landing propulsion isidle during horizontal cruise.

FIGS. 1 and 2 illustrate an aerial vehicle 100, in accordance with anembodiment of the disclosure. The illustrated embodiment of aerialvehicle 100 is a vertical takeoff and landing (VTOL) unmanned aerialvehicle (UAV) that includes separate propulsion units 106 and 112 forproviding horizontal and vertical propulsion, respectively. Aerialvehicle 100 is a fixed-wing aerial vehicle, which as the name implies,has a wing assembly 102 that can generate lift based on the wing shapeand the vehicle's forward airspeed when propelled horizontally bypropulsion units 106. FIG. 1 is a perspective view illustration ofaerial vehicle 100 operating during a vertical takeoff or landing withthe propeller blades of vertical propulsion units 112 deployed toprovide vertical propulsion. FIG. 2 is a plan view illustration ofaerial vehicle 100 operating in a horizontal cruise mode with thepropeller blades of vertical propulsion units 112 idle (i.e., notspinning) and stowed to reduce a drag profile during forward motion. Asillustrated, the propeller blades of vertical propulsion units 112 arestowed and passively align for reduced drag due to wind resistanceresulting from the forward motion of aerial vehicle 100. In contrast,the propeller blades of vertical propulsion units 112 are deployed inFIG. 1 due to centrifugal forces when vertical propulsion units 112 arespinning.

The illustrated embodiment of aerial vehicle 100 has an airframe thatincludes a fuselage 104 and wing assembly 102. In one embodiment,fuselage 104 is modular and includes a battery module, an avionicsmodule, and a mission payload module. These modules may be detachablefrom each other and mechanically securable to each other to contiguouslyform at least a portion of the fuselage or main body.

The battery module includes a cavity for housing one or more batteriesfor powering aerial vehicle 100. The avionics module houses flightcontrol circuitry of aerial vehicle 100, which may include a processorand memory, communication electronics and antennas (e.g., cellulartransceiver, wifi transceiver, etc.), and various sensors (e.g., globalpositioning sensor, an inertial measurement unit (IMU), a magneticcompass, etc.). The mission payload module houses equipment associatedwith a mission of aerial vehicle 100. For example, the mission payloadmodule may include a payload actuator for holding and releasing anexternally attached payload. In another embodiment, the mission payloadmodule may include a camera/sensor equipment holder for carryingcamera/sensor equipment (e.g., camera, lenses, radar, lidar, pollutionmonitoring sensors, weather monitoring sensors, etc.).

As illustrated, aerial vehicle 100 includes horizontal propulsion units106 positioned on wing assembly 102, which can each include a motor, amotor rotor with shaft, and propeller blades, for propelling aerialvehicle 100 horizontally. The illustrated embodiment of aerial vehicle100 further includes two boom assemblies 110 that secure to wingassembly 102. Vertical propulsion units 112 are mounted to boomassemblies 110. Vertical propulsion units 112 can each also include amotor, a motor rotor with shaft, and propeller blades, for providingvertical propulsion. As mentioned above, vertical propulsion units 112may be used during a hover mode where aerial vehicle 100 is descending(e.g., to a delivery location), ascending (e.g., following a delivery),or maintaining a constant altitude. Stabilizers 108 (or tails) may beincluded with aerial vehicle 100 to control pitch and stabilize theaerial vehicle's yaw (left or right turns) during cruise. In someembodiments, during cruise mode vertical propulsion units 112 aredisabled and during hover mode horizontal propulsion units 106 aredisabled. In other embodiments, vertical propulsion units 112 are merelypowered low during cruise mode and/or horizontal propulsion units 106are merely powered low during hover mode.

During flight, aerial vehicle 100 may control the direction and/or speedof its movement by controlling its pitch, roll, yaw, and/or altitude.Thrust from horizontal propulsion units 106 is used to control airspeed. For example, the stabilizers 108 may include one or more rudders108 a for controlling the aerial vehicle's yaw, and wing assembly 102may include elevators for controlling the aerial vehicle's pitch and/orailerons 102 a for controlling the aerial vehicle's roll. As anotherexample, increasing or decreasing the speed of all the propeller bladessimultaneously can result in aerial vehicle 100 increasing or decreasingits altitude, respectively.

Many variations on the illustrated fixed-wing aerial vehicle arepossible. For instance, aerial vehicles with more wings (e.g., an“x-wing” configuration with four wings), are also possible. AlthoughFIGS. 1 and 2 illustrate one wing assembly 102, two boom assemblies 110,two horizontal propulsion units 106, and six vertical propulsion units112 per boom assembly 110, it should be appreciated that other variantsof aerial vehicle 100 may be implemented with more or less of thesecomponents.

It should be understood that references herein to an “unmanned” aerialvehicle or UAV can apply equally to autonomous and semi-autonomousaerial vehicles. In a fully autonomous implementation, all functionalityof the aerial vehicle is automated; e.g., pre-programmed or controlledvia real-time computer functionality that responds to input from varioussensors and/or pre-determined information. In a semi-autonomousimplementation, some functions of an aerial vehicle may be controlled bya human operator, while other functions are carried out autonomously.Further, in some embodiments, a UAV may be configured to allow a remoteoperator to take over functions that can otherwise be controlledautonomously by the UAV. Yet further, a given type of function may becontrolled remotely at one level of abstraction and performedautonomously at another level of abstraction. For example, a remoteoperator may control high level navigation decisions for a UAV, such asspecifying that the UAV should travel from one location to another(e.g., from a warehouse in a suburban area to a delivery address in anearby city), while the UAV's navigation system autonomously controlsmore fine-grained navigation decisions, such as the specific route totake between the two locations, specific flight controls to achieve theroute and avoid obstacles while navigating the route, and so on.

FIGS. 3, 4, 5, 6A, and 6B are different views illustrating variousaspects of a subassembly 300 of a propulsion unit (e.g., a verticalpropulsion unit 112) for coupling propeller blades to a motor rotor, inaccordance with an embodiment of the disclosure. FIGS. 3 and 6A are topperspective views, FIG. 4 a bottom perspective view, and FIG. 5 is anexploded view of subassembly 300. The illustrated embodiment ofsubassembly 300 includes a motor rotor 305, a clip-in base mount 310, aholder base 315, a holder cap 320, pivot mounts 325, stop blocks 330,propeller blades 335, and a mechanical fastener 317. The illustratedembodiment of clip-in base mount 310 includes a raised alignment ring340, cam grooves 345, and detents 350 (see FIG. 6B). The illustratedembodiment of holder base 315 includes holding cams 355, a spring cavity360, and a spring 365 (see FIG. 4). The illustrated embodiment ofpropeller blades 335 includes a proximal base 370, a hole 375 inproximal base 370, and a distal tip 380 (see FIGS. 3 and 5). The motorrotor 305 includes a cover plate 307 connected to a shaft 309 both ofwhich rotate together.

Referring to FIG. 5, pivot mounts 325 are implemented as bearings 326that slide through holes 375 in propeller blades 335 and mate withrecesses 510 in holder cap 320. Bearings 326 constrain the motion ofpropeller blades 335 to a rotation about offset pivoting axes 505.Offset pivoting axes 505 are substantially parallel to, but physicallyoffset from, a central rotational axis 306 about which motor rotor 305turns. Bearings 326 extend between holder base 315 and holder cap 320along offset pivoting axes 505. Holes 375 in proximal bases 370 slideover and fit around bearings 325. In the illustrated embodiment,bearings 326 are cylindrical bosses attached to holder base 315, whichsecure into recesses 510 in holder cap 320. In one embodiment, holderbase 315 along with bearings 326 is fabricated of metal (e.g., aluminum)while propeller blades 335 are fabricated of plastic. In one embodiment,the holder base 315 along with bearings 326 is anodized and/or coatedwith polytetrafluoroethylene (PTFE) to improve wear characteristics. Inother embodiments, one or more of the components may be fabricated ofcarbon fiber reinforced polymer.

Holder cap 320 operates not only to support the top sides of bearings326 but also clamps propeller blades 335 between holder base 315 andholder cap 320. In the illustrated embodiment, a single mechanicalfastener 317 threads into a single female threaded boss on holder cap320 along central rotational axis 306 to provide the clamping force. Inother embodiments, bearings 326 may be implemented as female threadedbosses and a pair of mechanical fasteners may be threaded through holdercap 320 into the threaded bosses along offset pivoting axes 505 (e.g.,see FIG. 7). The length of bearings 326 relative to the thickness ofproximal bases 370 of propeller blades 335 is selected to allowpropeller blades 335 to freely pivot about offset pivoting axes 505without permitting undue slop or dihedral bending.

While motor rotor 305 is spinning about central rotational axis 306,propeller blades 335 pivot about offset pivoting axes 505 to theirdeployed position. Stop blocks 330 are mounted between propeller blades335 at different circumferential positions to limit the amount ofpivoting of propeller blades 335 in either rotational direction aboutoffset pivoting axes 505. When motor rotor 305 commences rotation atinitial spin up, stop blocks 330 push against the trailing edge ofpropeller blades 335 to initiate their rotational motion about centralrotational axis 306. Once propeller blades 335 are spinning, centrifugalforces takeover and propeller blades 335 pivot to their deployedpositions due to the offset position of pivot mounts 325. In theillustrated embodiment, the surfaces of stop blocks 330 that contactpropeller blades 335 have curvatures that mate to the local curvaturesof propeller blades 335 to distribute the forces on propeller blades 335over a larger area. In one embodiment, the contacting surfaces of stopblocks 330 are fabricated of a material that is softer than propellerblades 335 to reduce dents on or damage to propeller blades 335. Forexample, stop blocks 330 may have a rubberized coating, a plasticcoating, or otherwise.

While motor rotor 305 is idle (i.e., not spinning) during cruising ofaerial vehicle 100, wind resistance causes propeller blades 335 to pivotabout offset pivoting axes 505 to the stowed position. The windresistance and free rotational motion of propeller blades 335 at pivotmounts 325 allows propeller blades 335 to passively align with the windto reduce their cross-sectional area heading into the wind therebyreducing their drag profile. Stop blocks 330 are sized such that thegiven stop block 330 that comes to a rest in the downwind position alsoserves to prevent the aerodynamic surfaces (e.g., lifting surfaces,trailing edge, leading edge, etc.) of propeller blades 335 fromcontacting each other in the stowed position. This prevents binding,marring, or other damage to the aerodynamic surfaces on propeller blades335. Although FIGS. 4 and 5 illustrate stop blocks 330 integrated intoholder base 315, in other embodiments, the stop blocks may be integratedinto the holder cap (e.g., see FIGS. 8A and 8B). In yet otherembodiments, the stop blocks may be formed into the proximal bases 370of propeller blades 335 (not illustrated).

The illustrated embodiment of subassembly 300 is a clip-in embodimentthat attaches propeller blades 335 to motor rotor 305 by hand in thefield without the need of a tool. Accordingly, holder base 315 is alsoreferred to as a clip-in rotor cap 315 since it clips onto and caps overthe motor rotor 305. Referring to FIGS. 4, 6A and 6B, clip-in rotor cap315 (a.k.a. holder base 315), includes holding cams 355 that mate to camgrooves 345 disposed in clip-in base mount 310. Holding cams 355 aremechanically linkages that slide in cam grooves 345. In the illustratedembodiment, each holding cam 355 is a sort of foot or L-shaped bracketthat protrudes from the backside of clip-in rotor cap 315 that facesclip-in base mount 310. As illustrated in FIG. 6A, clip-in rotor cap 315detachably clips (mates) into clip-in base mount 310 with a three partmotion: (1) align holding cams 355 to cam grooves 345 and push thecomponents together to overcome the repelling force from spring 365, (2)twist clip-in rotor cap 315 relative to clip-in base mount 310, and (3)release clip-in rotor cap 315 such that spring 365 forces holding cams355 into detents 350. With reference to FIGS. 4 and 5, spring 365 isdisposed within spring cavity 360 and secured in place by mechanicalfastener 317 at the tight curl end of spring 365. Alignment ring 340(FIG. 6B) extends around the perimeter of clip-in base mount 310 andprovides an alignment structure and shear force retention for clip-inrotor cap 315.

In one embodiment, clip-in rotor cap 315 and clip-in base mount 310 aredesigned to provide automatic “pop off” separation in the event of apropeller blade collision with a physical object having a thresholdmass. For example, in one embodiment, detents 350 have beveled stops(only abrupt 90 degree stops 351 are illustrated) and the spring rate ofspring 365 selected such that a sufficient rotational force willseparate clip-in rotor cap 315 along with propeller blades 335 frommotor rotor 305 and clip-in base mount 310. The direction of rotationalseparation of holding cams 355 out of grooves 345 is selected to beopposite the rotational direction of motor rotor 305 during operation.In this manner, impact of propeller blade 335 with a stationary physicalobject can provide the correct impulse force and rotational direction toseparate clip-in rotor cap 315. The angle of the beveled stop (alongwith its slope direction) and the spring rate may be select such thatpop off occurs before propeller blades 335 are damaged by impact orbefore severe injury to a human hand occurs should the physical objectimpacted be a hand or finger. As illustrated in the embodiment of FIGS.6A and 6B, clip-in base mount 310 is secured to motor rotor 305 withmechanical fasteners (e.g., four) through screw holes 605.

FIG. 7 is a perspective view illustration of another subassembly 700 forcoupling propeller blades 335 to a motor rotor 705, in accordance withan embodiment of the disclosure. The illustrated embodiment ofsubassembly 700 includes motor rotor. 705, a holder base 715, a holdercap 720, pivot mounts 725, and mechanical fasteners 717 and 718.Subassembly 700 operates in a similar manner as subassembly 300 tosecure propeller blades 335 to motor rotor 705 using pivot mounts 725 atoffset pivoting axes 730 that are substantially parallel to but offsetfrom central rotational axis 740. However, pivot mounts 725 are bearingsfabricated from female threaded bosses extending from holder base 715.Holder cap 720 is secured over the female threaded bosses withmechanical fasteners 717 that thread through holder cap 720 into thefemale threaded bosses. Although motor rotor 705 is illustrated ashaving a hub, spoke, and rim cover plate, it should be appreciated thatthe cover plates of motor rotors 305 or 705 may assume this shape orotherwise.

FIGS. 8A and 8B are perspective view illustrations of a holder cap 800,in accordance with an embodiment of the disclosure. Holder cap 800includes integrated stop blocks 805. Holder cap 800 may be used inconnection with either of subassemblies 300 or 700 to replace theirillustrated holder caps 320 or 720, respectively.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. An aerial vehicle, comprising: an airframe; ahorizontal propulsion unit mounted to the airframe and oriented toprovide horizontal propulsion to the aerial vehicle; and a verticalpropulsion unit mounted to the airframe and oriented to provide verticalpropulsion to the aerial vehicle, the vertical propulsion unitincluding: a motor rotor that spins about a central rotational axis;propeller blades each having a proximal base and a distal tip; and pivotmounts each coupling the proximal base of a corresponding one of thepropeller blades to the motor rotor, wherein the propeller blades eachfreely pivot at the proximal base about a corresponding offset pivotingaxis that is offset from the central rotational axis of the motor rotor,wherein the propeller blades pivot to a deployed position under acentrifugal force when the motor rotor of the vertical propulsion unitis spinning to provide the vertical propulsion and pivot to a stowedposition due to a wind resistance resulting from a forward motion of theaerial vehicle when the motor rotor of the vertical propulsion unit isnot spinning.
 2. The aerial vehicle of claim 1, wherein the stowedposition has a reduced drag profile during the forward motion of theaerial vehicle.
 3. The aerial vehicle of claim 2, wherein the verticalpropulsion unit further includes: stop blocks each mounted between thepropellers blades at a different circumferential position about thecentral rotational axis, the stop blocks positioned to limit an amountof pivoting of the propeller blades in either rotational direction aboutthe pivot points and to prevent aerodynamic surfaces of the propellerblades from contacting each other in the stowed position.
 4. The aerialvehicle of claim 3, wherein one or more surfaces of the stop blocks thatcontact the propeller blades are fabricated of a material that is softerthan the propeller blades.
 5. The aerial vehicle of claim 1, wherein thevertical propulsion unit further includes: a holder base secured to themotor rotor; a holder cap; and bearings forming the pivot mounts, thebearings each extending between the holder base and the holder cap alongthe corresponding offset pivoting axis, wherein a hole in the proximalbase of each of the propeller blades fits around a corresponding one ofthe bearings and the propeller blades are clamped between the holderbase and the holder cap.
 6. The aerial vehicle of claim 5, wherein thebearings comprise female threaded bosses extending from the holder base,and wherein the holder cap is secured over the female threaded bosseswith mechanical fasteners that thread through the holder cap into thefemale threaded bosses.
 7. The aerial vehicle of claim 5, wherein theholder cap is secured to the holder base with a mechanical fasteneraligned with the central rotational axis.
 8. The aerial vehicle of claim5, wherein the holder base comprises a clip-in rotor cap, and whereinthe vertical propulsion unit further includes: a clip-in base mountdisposed on the motor rotor, wherein the clip-in rotor cap is shaped tomate with and detachably clip into the clip-in base mount.
 9. The aerialvehicle of claim 8, wherein the clip-in rotor cap includes: holding camsthat mate to one or more cam grooves disposed in the clip-in base mount;and a spring cavity; and a spring disposed in the spring cavity thatasserts a repelling force between the clip-in base mount and the clip-inrotor cap when the clip-in rotor cap is mated to the clip-in base mount.10. The aerial vehicle of claim 9, wherein the clip-in base mountcomprises: a raised alignment ring extending at least partially around aperimeter of the clip-in base mount; the one or more cam groovesdisposed along the raised alignment ring; and detents disposed in theone or more cam grooves to provide resistance to a rotational separationof the holding cams from the one or more cam grooves.
 11. The aerialvehicle of claim 10, wherein a beveling of the detents along with adirection of the rotational separation of the holding cams from the oneor more grooves enables a pop off separation of the clip-in rotor capand the propeller blades from the motor rotor in an event that aphysical object of a threshold mass impacts one or more of the propellerblades when the propeller blades are spinning.
 12. A propulsion unit,comprising: a motor rotor that spins about a central rotational axis;propeller blades each having a proximal base and a distal tip; pivotmounts formed by bearings that each couple the proximal base of acorresponding one of the propeller blades to the motor rotor, whereinthe propeller blades each freely pivot at the proximal base about acorresponding offset pivoting axis that is offset from the centralrotational axis of the motor rotor; a holder base securable to the motorrotor; a holder cap, wherein the bearings each extending between theholder base and the holder cap along the corresponding offset pivotingaxis, wherein a hole in the proximal base of each of the propellerblades fits around a corresponding one of the bearings; and a clip-inbase mount disposed on the motor rotor, wherein the holder basecomprises a clip-in rotor cap shaped to mate with and detachably clipinto the clip-in base mount.
 13. The propulsion unit of claim 12,wherein the propeller blades pivot between a deployed position thatprovides propulsion and a stowed position having a reduced drag profiledue to wind resistance that is perpendicular to the central rotationalaxis.
 14. The propulsion unit of claim 13, further comprising: stopblocks each mounted between the propellers blades at a differentcircumferential position about the central rotational axis, the stopblocks positioned to limit an amount of pivoting of the propeller bladesin either rotational direction about the pivot points and to preventaerodynamic surfaces of the propeller blades from contacting each otherin the stowed position.
 15. The propulsion unit of claim 12, wherein thepropeller blades are clamped between the holder base and the holder cap.16. The propulsion unit of claim 15, wherein the bearings comprisefemale threaded bosses extending from the holder base, and wherein theholder cap is secured over the female threaded bosses with mechanicalfasteners that thread through the holder cap into the female threadedbosses.
 17. The propulsion unit of claim 15, wherein the holder cap issecured to the holder base with a mechanical fastener aligned with thecentral rotational axis.
 18. The propulsion unit of claim 12, whereinthe clip-in rotor cap includes: holding cams that mate to one or morecam grooves disposed in the clip-in base mount; and a spring cavity; anda spring disposed in the spring cavity that asserts a repelling forcebetween the clip-in base mount and the clip-in rotor cap when theclip-in rotor cap is mated to the clip-in base mount.
 19. The propulsionunit of claim 18, wherein the clip-in base mount comprises: a raisedalignment ring extending at least partially around a perimeter of theclip-in base mount; the one or more cam grooves disposed along theraised alignment ring; and detents disposed in the one or more camgrooves to provide resistance to a rotational separation of the holdingcams from the one or more cam grooves.
 20. The propulsion unit of claim19, wherein a beveling of the detents along with a direction of therotational separation of the holding cams from the one or more groovesenables a pop off separation of the clip-in rotor cap and propellerblades from the motor rotor in an event that a physical object of athreshold mass impacts one or more of the propeller blades when thepropeller blades are spinning.
 21. The propulsion unit of claim 12,wherein the corresponding offset pivoting axis of each of the propellerblades is parallel to the central rotational axis of the motor rotor.22. A propulsion unit, comprising: a motor rotor that spins about acentral rotational axis; propeller blades each having a proximal baseand a distal tip; pivot mounts each coupling the proximal base of acorresponding one of the propeller blades to the motor rotor, whereinthe propeller blades each freely pivot at the proximal base about acorresponding offset pivoting axis that is offset from the centralrotational axis of the motor rotor; and stop blocks each mounted betweenthe propellers blades at a different circumferential position about thecentral rotational axis, the stop blocks positioned to limit an amountof pivoting of the propeller blades in either rotational direction aboutthe pivot points and to prevent aerodynamic surfaces of the propellerblades from contacting each other in a stowed position, wherein one ormore surfaces of the stop blocks that contact the propeller blades arefabricated of a material that is softer than the propeller blades.